In a case where a thermoplastic resin composition is variously used for housings of a personal computer and a display device, an electronic device material, automotive exterior and interior parts, and the like, there sometimes occurs a problem that it is difficult to release generated heat because plastic is less thermally conductive than an inorganic substance such as a metal material. In order to solve the problem, an attempt has been extensively made to obtain a highly thermally conductive resin molded article by blending, with a thermoplastic resin, a highly thermally conductive inorganic substance in a large amount. In order to obtain the highly thermally conductive resin molded article, it is necessary to blend, with a resin, a highly thermally conductive inorganic substance such as graphite, carbon fiber, a low melting metal, alumina, or aluminum nitride normally in an amount of not less than 30% by volume and further in an amount of as many as 50% by volume.
Of these resin compositions, a resin composition in which graphite, carbon fiber, or a low melting metal is used allows obtainment of a resin molded article which is comparatively highly thermally conductive. However, the resulting resin molded article is electroconductive. As such, it is difficult to differentiate such a resin molded article from a metal, and the resin molded article has limited uses.
In contrast, a resin composition in which alumina is used can be both electrically insulative and highly thermally conductive. However, there is a problem that (i) it is difficult to meet requirements for a reduction in weight of a portable electronic device because alumina, which is more dense than a resin, gives a resulting resin composition a high density and (ii) alumina cannot make a significant improvement in thermal conductivity of the resulting resin composition. Use of aluminum nitride makes it possible to obtain a composition which has a comparatively high thermal conductivity, although, hydrolyzability of aluminum nitride and the like is a matter of concern.
Further, such a thermoplastic resin composition which is highly dense with a filler has a problem that the high filler content causes a large reduction in injection moldability, thereby making it extremely difficult to carry out injection molding of the thermoplastic resin composition with a mold which has a practical shape or a mold which has a pin gate. In order to attain a higher injection moldability of a highly thermally conductive thermoplastic resin which is highly dense with a filler, for example, Patent Literature 1 discloses a method for adding a liquid organic compound at room temperature. However, such a method has at least a problem that a mold is contaminated by a liquid organic compound which is bled out during injection molding. Various attempts for a better moldability as well as these techniques have failed to provide an effective method.
Note that, in a case where such a highly thermally conductive thermoplastic resin composition is to be used for an exterior resin of a portable electronic device, its high thermal conductivity allows heat transmission to outside directly from a part in which high heat is locally generated, for example, in a CPU provided in the portable electronic device. This would result in that the outside of the portable electronic device immediately have an extremely high temperature, which would cause a problem such as a burn on a human body when the portable electronic device is in contact with the human body.
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2003-41129 A (Publication Date: Feb. 13, 2003)